A wideband multiple channel frequency converter is particularly useful in the field of wireless communication technology, and in fact, is particularly useful in wireless base station systems. In the current expanding wireless communication environment, systems require both high data rates and high processing gains. Wideband transceivers are essential to realize these features demanded by wireless communication systems. In fact, it is believed that the next generation of wireless transceivers, for both multimedia applications and for wireless networking, must be wideband systems.
In addition, a large variety of digital systems have recently emerged in the wireless market. To combine the emerging digital systems with existing analog systems, and to reduce the cost of a wireless system, a new technology, i.e., a wideband software-defined radio base station system, is developing. Essentially, in a software-defined radio base station transmitter, digital base-band channels are processed separately. All channels are then combined together, and the combined signal (i.e., a wideband multiple channel signal), after a digital to analog conversion, is then up-converted to RF by a wideband multiple channel frequency converter In some cases however, a second intermediate up-conversion stage may also be required to up-convert the signal to RF.
A wideband multiple channel frequency converter with very low inter-modulation components (for example, -60 dBc or even lower) is essential to realize such a wideband multiple channel transceiver. Thus, it would be beneficial to develop a system for realizing a wideband multiple channel frequency converter with improved performance, with low inter-modulation components and wherein only desired signals are output from the converter.
Consider the following situation for a frequency up-converter (it should be noted that while the discussion below is focused on a frequency up-converter, it will appreciated that the discussion is equally applicable to down-converters.). A wideband multiple channel intermediate frequency (IF) signal is input to an up-converter. The performance requires an up-converted RF signal at the output port of the converter, with low inter-modulation components which will allow a band-pass filter to easily filter out the undesired components from the output signal. In analyzing the response of the up-converter to the input wideband signal, two line signals are illustrated in FIG. 1 to represent the wideband signal.
A conventional implementation of a frequency converter utilizes a mixer. Considering a wideband signal, i.e., two line signals f.sub.1 and f.sub.2, input to the mixer, the local oscillator signal (LO) is f.sub.0, and following condition is satisfied: EQU f.sub.0 &gt;&gt;f.sub.2, f.sub.1 &gt;&gt;(f.sub.2 -f.sub.1)(f.sub.2 &gt;f.sub.1 assumed)
resulting in a simplified output frequency spectrum of a conventional frequency converter, as shown in FIG. 1 (note that FIG. 1 shows only the components containing f.sub.0 and neglects those components containing 2f.sub.0, 3f.sub.0. . . ). In FIG. 1 we see that there are many inter-modulation components, together with a possible local oscillator (LO) leakage signal. The only signals desired to be output from the frequency converter are (f.sub.0 +f.sub.1) and (f.sub.0 +f.sub.2) (considering the "up branch", for example).
Traditional technologies have not solved the inter-modulation (e.g., f.sub.0 -4f.sub.2 +3f.sub.1, f.sub.0 -3f.sub.2 +2f.sub.1, etc.) and local oscillator (LO) signal (f.sub.0) leakage problems illustrated in FIG. 1. Although it may be known that inter-modulation components can be reduced by reducing the power level input to a mixer for wideband mixers, there is typically local oscillator (LO) signal leakage to the output RF port, and, if the power level of an input signal is reduced to a low level, the local oscillator (LO) leakage at the RF port may be even higher than the desired output RF signal.
An example of such an occurrence is illustrated with respect to the conventional M/A-COM wideband mixer MD-123 PIN, the parameters of which are listed below:
Frequency range: RF, LO: 0.01-3 GHz IF: 0.01-3 GHz PA1 Typical two-tone IM ratio: -56 dBc when each tone is -10 dBm at the input PA1 Conversion loss: 8 dB PA1 LO power needed: 10 dBm min PA1 Isolation: LO to RF: 25 dB; LO to IF: 25 dB; RF to IF 25 dB
As noted in the discussion above, inter-modulation components may typically be reduced by reducing the power level input to the mixer. For example, if the input signal power to the MD- 123 PIN is reduced to -12 dBm, the inter-modulation components will be reduced sufficiently to -60 dBc. However, the desired signal output at the RF port will be -20 dBm, and the local oscillator LO leakage at the RF port will be -15 dBm (i.e., the local oscillator (LO) leakage is actually 5 dB higher than the desired RF signal).
The present invention is therefore directed to the problem of developing a wideband multiple channel frequency converter which allows only the desired signals to be converted and output, and which rejects or significantly reduces other unwanted components, including both inter-modulation components and local oscillator leak components.